Visual excitation is initiated by the absorption of a photon by the 11-cis retinal chromophoric group in the visual pigment, rhodopsin. This event is followed by a sequence of conformational changes in the visual protein and by the cis yields trans isomerization of retinal. The objective of this proposal is to determine in detail the molecular mechanism for these events through the use of resonance Raman spectroscopy. This objective will be approached in the following ways: (1) Time-resolved resonance Raman spectroscopy will be used to examine the spectra and kinetics of rhodopsin's earliest photolytic intermediates, bathorhodopsin and lumirhodopsin. (2) Raman spectra of visual pigment analogues regenerated with isotopic derivatives of retinal will be used to assign these Raman spectra. (3) The vibrational spectra of model polyenes, polyene aldehydes and their isotopic derivatives will be used to develop and refine a QCFF-pi normal mode calculation. The comparison of visual pigment Raman spectra with the corresponding model compound data ahould reveal when the complete cis yields trans isomerization has occurred and what intermediate conformations of retinal are photochemically important. This comparison will be facilitated by our vibrational normal mode calculations. (4) The conformational predications of this work for retinal photochemistry will be tested through Raman studies of visual pigments with altered proteins. In particular, resonance Raman microscopy will be used to examine individual rod and cone cells with different photochemical characteristics.